Nerve conduction studies and evoked potential measurements are now commonly made in clinical practice and in research to evaluate nervous system functions. To measure the evoked potentials stimulated by electrical pulses, surface measurement electrodes are customarily positioned on the scalp or skin over peripheral nerves. The electrical potentials received by these electrodes are detected and analyzed by sensitive recording equipment. To stimulate a response in the nervous system, stimulation electrodes are applied to the skin of the subject at a position remote from the measurement electrodes, typically on an arm or leg, and a pulse of either constant voltage or constant current magnitude is then applied to the individual between the two stimulation electrodes.
In performing sensory nerve conduction studies, it is necessary to use relatively high levels of voltage (e.g., hundreds of volts) and/or current (e.g., tens of milliamperes) to depolarize the nerve and elicit a response. Once the nerve is depolarized, the compound nerve action potential (CNAP) travels along the nerve in both directions. The velocity of this response is an important parameter in the diagnosis of various neuropathies.
To determine the velocity of this CNAP, recording electrodes are placed directly over the nerve being stimulated. By connecting these recording electrodes to a physiological amplifier, both the amplitude and the latency (the time it takes for the response to reach the recording electrodes) can be determined. Where neuropathies are present, the amplitudes can be less than a microvolt. In addition, for short nerve conduction distances, the latencies can be on the order of one to two milliseconds. The nerve conduction velocity is calculated by taking the ratio of the distance of conduction to the latency time.
A significant difficulty is encountered in measuring the response potentials because the electrical stimulator produces a large electric field potential. This field potential reaches the recording electrodes almost instantly and generates a response commonly referred to as the “stimulus artifact.” This artifact is problematic for two reasons. First, since the response potential is so small, the amplifier gains are typically set very high. Therefore, the large potential from the stimulator drives various stages of the amplifier into saturation. As a result, the amplifier may still be recovering from the saturation condition and have not returned to baseline when the response potential arrives. This can obscure the “take-off” point of the response and introduce an error in the velocity calculation. Secondly, the physiological amplifier is usually AC coupled through a high pass filter capacitor so that DC offset potentials on the electrodes can be removed from the signal picked up by the electrodes. However, the large potential from the stimulus artifact can inject a charge on this filter capacitor, and the resulting RC time constant can, again, create a delay in the return of the amplifier to baseline.